I don't think an endocrinologist workup would be the first things to look into considering your symptoms.
There is no one condition "dysautonomia," so I'm unsure what your cardiologist means about treating POTS the same as dysautonomia. Different forms of dysautonomia or secondary dysautonomias (e.g., autoimmune autonomic ganglionopathy, pure autonomic failure, Sjogren's disease, mitochondrial disease) have different treatments. Is it possible for you to get a second opinion from a doctor with more knowledge/experience with dysautonomia?
POTS is rarely diagnosed and managed by a general practitioner. Usually an EP cardiologist (electrophysiologist) does the tilt table test and other diagnostics needed. If a more broad testing of the autonomic nervous system is needed, a neurologist may get involved, usually a Neuro-muscular specialist, sometimes one who has specialty in autonomic neurology.
My recommendation would be to pursue further autonomic testing with a more knowledgeable doc.
Welcome to the dysautonomia forum! I am sorry you are having a very rough time of it.
What tests have they run so far? Have you had a complete work up from an endocrinologist, since you have hormonal issues (polycystic and hypothyroidism)?
Have they done any MRIs of your brain, or dynamically done pituitary MRIs with your symptoms? The hypothalamus gland in your brain is responsible for temperature regulation and your pituitary gland is not far away.
Also, have they checked your cervical spine with MRI to see if there is any encroachment or damage there (in regards to your breathing)? Have they looked to make sure there is no issue with your spine in regards to your gastrointestinal system changes?
Have they made sure you were not anemic, since you had low oxygen with the pulseoximeter and also since you have dizziness even when your blood pressure is fine and headaches?
What tests has a gastrointestinal specialist run on you, if any?
Have they checked you for autoimmune problems, such as rheumatoid arthritis, etc.?
Do you have an HMO? If so, see if they have a nurse clinical advisor that can try to assist you to get the help you need.